Many providers of web sites rely on online advertisements as a source of revenue. The providers may rely on third-party advertisement servers to supply the advertisements that are included on the web pages of their web site. When a web page is generated, advertisement space may be allocated on the web page with a hyperlink that references the advertisement server. When the web page is displayed by a browser, the browser resolves the hyperlink by sending a request to the advertisement server. The advertisement server identifies an appropriate advertisement and returns to the browser the advertisement, which may include a link to a web page relating to what is being advertised. The browser then displays the advertisement as part of the web page. The provider of the web page may be compensated for the advertisement when a user selects the hyperlink of the advertisement.
To improve the chances of increasing revenue, a provider of web pages may crowd the web pages with advertisements. The crowding of web pages with advertisements may, however, have negative consequences. For example, mobile devices, such as handheld personal computers, cell phones, and personal digital assistants (“PDAs”), are becoming increasingly popular ways to access the Internet. Unfortunately, the experience of users in accessing web pages with many advertisements using these mobile devices has been less than satisfactory. First, because these mobile devices have small display areas, it is difficult to effectively display in such a small display area a web page with many advertisements. Second, because these mobile devices typically have a relatively slow communications link (e.g., via a cell phone network), it can take an unacceptably long time to download an accessed web page that is crowded with advertisements. Third, because the mobile devices have limited computing power, the rendering of a web page crowded with advertisements can also take an unacceptably long time.
As another example, many search engine services provide for searching for images that are accessible via web pages. These search engine services allow users to search for images that may be of interest to users. After a user submits a search request (i.e., a query) that includes search terms, the search engine service identifies images that may be related to those search terms. To quickly identify related images, the search engine services may maintain a mapping or index of keywords to images. This mapping may be generated by “crawling” the web (i.e., the World Wide Web) to identify the keywords of web pages that contain the images. To crawl the web, a search engine service may use a list of root web pages to identify all web pages that are accessible through those root web pages. Whenever the search engine service identifies an image on a web page, it adds the image to the mapping. Since many of the images of web pages are images that are part of advertisements, the mapping will contain images of advertisements, which are typically not of interest to users searching for images. When there are a significant number of images of advertisements, it takes longer for the search engine service to generate the mapping, the mapping consumes more system resources, it takes longer to access the mapping, and the search results may contain many images of advertisements, which are not of interest to the user.